Category:Exhibit Case Design
Designing a Conservation-Grade Case
- Design cases as protective enclosures. Take advantage of a well-designed case to control the microenvironment of sensitive collections. A case designed with the participation of an exhibit conservator is an efficient and often cost- effective way to meet conservation criteria for an object.
- Establish performance criteria. Determine what conservation features will be built into each case, and clearly identify performance criteria for each feature. Design the case to provide this performance.
- Build and test complex case designs as prototypes when possible. Modify the case design until acceptable performance is achieved.
- Provide detailed, explicit drawings and specifications. Inspect cases during fabrication to ensure that the fabricators stick to specifications and construction tolerances.
- Verify performance for the fully assembled case in its final location to ensure that conservation criteria have been met. Such testing should occur before object installation to allow for adjustments.
Conservation and Case Design
Displaying three-dimensional objects inside an exhibit case and framing two-dimensional materials provide protection that cannot be matched by open display. A sealed or ventilated enclosure creates a microenvironment that can be designed to fulfill a variety of conservation criteria. The choice between sealed and ventilated cases depends on the circumstances of the particular exhibit, the ambient exhibit environment, and the sensitivity of the objects.
The design of a conservation-grade case presents technical challenges requiring significant oversight by the designer and conservator. Creating the case involves control at three levels: design, fabrication, and testing. For a case to fulfill conservation criteria, functional requirements must be clearly and specifically stated. Conservation features should be discussed with the exhibit conservator early in the design process.
Conservation Grade-Exhibit Cases
An exhibit case protects an object from physical damage. A case also provides the opportunity to create a microclimate that satisfies stringent conservation criteria for vulnerable objects. A case, or even a well engineered frame, can be designed to meet different levels of one or more of the following conservation features:
- Protection from incidental touch
- Decreased threats of theft and vandalism (see TechNote: Security Case Glazing)
- Prevention of insect and rodent entry (see TechNote: Pest Management for Exhibit Areas)
- Filtration of dust and foreign substances (see TechNote: Filters for Ventilated Cases)
- Exposure to decreased levels of chemical pollution (see TechNotes: Sealing of Exhibit Cases and Exhibit Case Construction Materials)
- Removal or limitation of ultraviolet and infrared radiation (see TechNotes: Exhibit Case Lighting, and Selecting Exhibit Case Glazing)
- Buffering of collections from rapid changes in temperature and relative humidity or control of RH (see TechNotes: Environmental Control and Sealing of Exhibit Cases)
- Provision of a low-oxygen or inert gas atmosphere (see TechNotes: Sealing of Exhibit Cases and Environmental Control)
Although the style of wall and freestanding exhibit cases varies widely, preservation concerns focus on three elements:
- the display chamber
- the environmental maintenance chamber
- the exhibit lighting chamber
Every case has a display chamber that houses the objects. The design and material composition of this chamber are key to preservation-responsible design. Not every case, however, has maintenance and lighting chambers.
A separate environmental maintenance compartment is required when conservation criteria call for environmental or chemical pollutant control within a case. Exhibit lighting that originates in the case is conventionally mounted in a separate lighting chamber (or attic) with its own entry and adequate venting.
Prototype Production and Verifying Case Performance
Once the design for a conservation-grade case has been agreed upon, the drawings and specifications must be explicit enough for the fabricators to understand the construction details. If funding allows, fabricate a prototype of the design that can be fully tested and modified to correct design or fabrication deficiencies. This step is critical if the case incorporates complex conservation elements.
If a prototype is not constructed, one or two mid- production inspections are necessary to guarantee that the specified conservation features are included. Performance-testing is a necessary step when experimental or problematic designs are being proposed, new construction materials are used or when the fabricator does not have considerable experience with building conservation grade cases.
Assess the lighting equipment, heat buildup from lamps and fixtures, humidity controlling system, internal air mixing and air seal. Final adjustments and alterations may be required for the case to function as intended. Because final evaluation and testing are done on the assembled case in the exhibit space, the installation schedule must allow time for this activity. Certain design and performance features are of particular concern to the exhibit conservator:
- Overall adherence to design specifications
- Ease of object access and display chamber entry
- Ease of access to lighting and environmental maintenance chambers
- Adequate air communication between display and maintenance chambers
- Adequacy of seal and rate of air exchange between case interior and exterior
- Proper seating of gaskets and sealing of joints to control air leakage
- Use of nonhazardous materials; isolation layers and barrier coatings between problematic construction materials and display objects
- Levels of volatile organic compounds
- Successful functioning of environmental control systems, if included
- Acceptable levels of visible, ultraviolet, and infrared radiation in the display chamber
- Acceptable levels of heat buildup from exhibit lighting
These features will be addressed in further detail later in this section.
Case Stability, Security, and Access
- Construct a physically stable, structurally secure case. Limit vibration by using movement-dampening devices. When floor or wall attachment is not possible, include space for a weight ballast to prevent jarring and tipping.
- Provide appropriate security features. Choose from security options to include the level of protection that the design team considers prudent. The case strength, resistance, and security devices should match the projected threat from vandalism and theft.
- Ensure practical access design for curatorial entry. Incorporate doors or other practical access options in the case design. Ensure that a single person can enter the case and remove artifacts with ease and in a short amount of time.
To provide basic protection, an exhibit case must be well fabricated and structurally stable. It should isolate and protect the objects from the physical forces of shock, vibration, and, in some regions, earthquakes. To limit the movement and vibration of case contents, do not connect cases to walls or ceilings that experience vibration from visitor movement, HVAC systems, or other equipment.
Design the case and floor interface so that cleaning equipment does not jar the case interior. Anchor freestanding cases and provide additional interior ballast for top-heavy pedestal cases to prevent tipping. In earthquake-prone regions, secure exhibit cases with specialized anchoring systems engineered to absorb severe lateral movement.
Different levels of security against theft and vandalism can be designed into an exhibit case. Objects of moderate to high monetary value, objects that are likely targets of theft, or objects displayed in unattended locations can be protected through careful case design. On the basis of a security risk assessment conducted early in the project, the exhibit team should identify the device or design feature that meets the physical security need. Given the tremendous range and cost of security equipment, obtain estimates before proceeding with case design. Available security features include the following:
- dual locking systems
- movement sensors and alarms
- shatterproof security glazing (seeTechNotes Exhibit Case Glazing)
- special glazing against forced entry, ballistic attack, and bomb blast. (see TechNotes Exhibit Case Glazing)
- concealed case screws and entry hardware.
- tamper-resistant hardware
- tamper-resistant hangers for frames.
In addition to providing security, an exhibit case also offers some protection during a natural or man-made disaster. Consider using fire- and water-resistant materials. Exhibit cases should protect their contents from fire exposure for at least 30 minutes and from the force of over- head sprinklers and fire department hoses.
When the case environment is controlled by a mechanical system, include internal heat- and combustion-sensors. When several such cases are part of an exhibit, connecting them to an early warning air-sampling smoke detection system may be more cost-effective.
Access to Display Objects
While discouraging unofficial access is a performance feature of any case, reasonable access must be provided for a variety of curatorial and maintenance functions, including:
- cleaning the case interior;
- maintenance of environmental control equipment located in the case;
- periodic object inspection and routine collections care procedures; and
- retrieval of objects for object rotation and emergencies.
When possible, ensure that each object is accessible without major disruption to adjacent objects. Provide sufficient access doors of adequate size.
Seek curatorial and conservation advice when determining the design of the access doors, their weight, and their locations. Rollers and temporary opening supports make oversized doors easier to open. In some walk-in cases, creating an interior pathway for access may be necessary.
When possible, the vitrines (bonnets) on freestanding cases should be of manageable size and weight to allow access by a single person; 30 inches square is the approximately the largest size for a lift-off acrylic vitrine. The vitrine top should never be so tall that it cannot be lifted safely over the objects inside the case. Lifting oversized glass panels is inadvisable
Sealed Exhibit Cases
- Use sealed display cases when appropriate. Determine which objects, if any, require protective micro-environments, and design cases accordingly. Design cases to avoid the risks presented from interior contaminates and from condensation due to exterior temperature change.
- Design well-sealed cases with tight joints and gaskets around all removable panels and entry doors. Choose construction materials that limit air exchange and, for climate-controlled case designs, materials that are not moisture-permeable. Well-sealed cases should allow no more than one complete air exchange every 72 hours. (see TechNotes: Sealing of Exhibit Cases)
- Use conservation-appropriate sealants. Minimize leaks with adequate gaskets and caulk. Always choose non-hazardous materials. (see TechNotes: Using Gaskets to Seal Exhibit Cases and Caulk Sealant to Seal Exhibit Cases)
- Test case performance. When possible, use leak detection equipment to identify air leaks and determine air exchange rates. Modify the case design or add caulk and gaskets to reduce leakage. (see TechNotes: Using Ultrasonic Leak Detection Equipment to Test Exhibit Case Seal)
Applications for Sealed Cases
Providing a microclimate in a well-sealed case is a low-cost alternative to controlling the entire exhibit space. Sealed cases are a particularly good choice when only a few exhibit objects are sensitive to humidity or when one or more objects require a different relative humidity. (see TechNotes: Environmental Control)
Enclosing papers, photographs, and textiles within tightly sealed frames can have the same benefits as putting them in sealed cases. A sealed frame incorporates an impermeable barrier as a backing board and uses a gasket material or tape to prevent air passage along the frame edges and glazing.
A well-sealed exhibit case limits the rate of air exchange between the display chamber and the ambient environment. The climate inside a sealed exhibit can be engineered to:
- buffer changes in temperature and relative humidity
- maintain a specific relative humidity
- provide a dust-free, insect-excluding display environment
- prevent external pollutants from deteriorating display objects
Construction of Sealed Cases
The degree of seal is measured in the number of air exchanges per hour. Most sealed museum cases provide a minimal to moderate rather than a hermetic seal. (See Exhibit Case Preservation Features and Controls) An unsealed exhibit case may undergo several exchanges of air per hour, while the exchange rate in a well-sealed case is as little as one complete air exchange every 72 hours or longer. (see TechNotes: Determining Exhibit Case Air Exchange Rates)
Higher degrees of seal reduce air exchange rates and improve the efficiency of the climate-control system within the case. To maximize performance of a well-sealed case, the design must incorporate these features:
- moisture-impermeable construction materials
- precise fitting of construction joints and seams
- conservation-safe caulk sealant along problematic construction joints and seams (see TechNotes: Caulk Sealant to Seal Exhibit Cases)
- conservation-appropriate gaskets on removable panels and doors.(see TechNotes: Using Gaskets to Seal Exhibit Cases)
Natural convection forces air into and out of small gaps or holes in the exterior of the case. Where case components meet, limit air movement through the use of tight joint construction and by sealing gaps with gaskets or caulk sealant.
Select gaskets that are appropriate for exhibit case applications. Among foam and extruded elastomer gaskets, there are many options. (see TechNotes: Using Gaskets to Seal Exhibit Cases) The foam must be of the proper density and thickness for successful compression by vitrine tops and doors. When possible, set foam gaskets into a channel or trough that measures 60% of the gasket's depth.
Curatorial or maintenance access doors require special engineering. Door hinges must allow clearance for the gasket dimensions. The density of the gasket material is important, as are its location, continuity, and dimensions. To prevent air leakage, it is critical that access doors and removable panels meet these requirements:
- attached with enough fasteners that compress the gasket material
- do not bow or deform under the pressure of the fasteners
- seated evenly against a uniform gasket surface
- fitted with a continuous gasket with no gap between sections
- do not bind against the gasket along the hinged side
Small door openings decrease the potential for case leakage. Some case designs have successfully used prefabricated, commercially available airtight hatches. Acrylic maritime yacht portals, for example, can be used as small access doors into maintenance chambers. Larger access doors require fastening devices every 18 to 24 inches and the use of crank-down or cam fasteners.
A well-sealed exhibit case should be tested for air leakage using an ultrasonic leak detector. (see TechNotes: Using Ultrasonic Leak Detection Equipment to Test Exhibit Case Seal) If leaks are identified, modifications can be made, including adjustments to fasteners or gaskets or the addition of caulking sealants to improve the case seal.
Even though air gaps are closed, a case which employs permeable materials such as unsealed wood products will allow moisture to migrate through the walls, floor, or ceiling. The rate of migration depends on the permeability of the material and the differential between interior and exterior relative humidity. Glazing materials such as acrylic (1/4 inch thick or greater) and glass have low diffusion rates that are suitable for well-sealed cases. Although moisture diffusion occurs slowly through plywood, particleboard, or drywall, these materials alone will not seal a case. (see TechNotes: Selecting Exhibit Case Glazing) Their permeability can be reduced by:
- adding a vapor barrier, such as a laminate of resin-based melamine sheet materials, metal foil, or polyethylene; or
- painting with a moisture barrier coating, paying strict attention to coating thickness requirements and film uniformity.
Consequences of Sealing Cases
As one consequence of sealing an exhibit case, emissions or offgassing from construction or finishing materials can become concentrated within the case. It is critical that only nonhazardous, non-emitting materials are used in the display and maintenance chambers or that a physical barrier is used to isolate potentially offgassing materials from the display chamber. (see TechNotes: Monitoring Pollutants Inside and Exhibit Case and Using Pollutant Absorbers Inside an Exhibit Case)
Sealed cases are also particularly susceptible to overheating due to improper lighting design or location next to a heating source. The lighting plan and ventilation design must prevent overheating. Similarly, placing a sealed case near heating or air-conditioning ducts or return vents or near a window can have an adverse impact on the microclimate inside the case.
Ventilated Exhibit Cases
- Use ventilated cases for appropriate applications. Select vented cases for use in an exhibit space with a good climate-control and pollutant-control system that functions 24 hours a day.
- Control the ventilation design of cases. Design well-sealed cases, and place an adequate number of vents to provide for air movement. Filter the vents to prevent dust, insects, and chemical pollutants from being drawn into the case.
- Use positive-pressure cases when appropriate. Museums with good climate-control systems may be able to use these cases, which are economical to build because they do not have to be well-sealed.
Applications for Ventilated Cases
A ventilated exhibit case is the alternative to a sealed exhibit case. Most conventional case designs have allowed ventilation through unsealed joints and gaps. Although this type of case is easy to construct, uncontrolled ventilation does not protect display objects from infiltration of dust, insects, or chemical pollutants. It is preferable, therefore, to construct a moderately sealed to well-sealed case and control ventilation intentionally through vents-also called ports.
A ventilated case has the same interior temperature and relative humidity as the ambient exhibit space, but it filters the incoming air to exclude dust, insects, and/or chemical pollutants. Ventilated cases are therefore appropriate only when temperature and relative humidity conditions throughout the museum or exhibit space meet the established conservation criteria for the displayed collections.
Ventilated and sealed cases can be used for the same exhibit. Frequently, most display cases are ventilated, while only a few well-sealed cases display objects that require specialized climates.
In some exhibits, conditioning the entire exhibit space may not be feasible or practical, even when all the collections are environmentally sensitive. In this situation, all cases should be designed as well-sealed, controlled microenvironments. Cases including these conservation features will cost more to design and produce; the exhibit budget should allow 20 to 40% more for case design and construction.
Ventilation vents, or ports, can be unobtrusively placed within decorative features such as reveals of the cabinetry, in recessed kick space, behind raised graphic panels, and on the backs and tops of cases. As in a sealed case, the intentionally reduced airflow through these cases makes the choice of nonhazardous construction and finishing materials an imperative.
Air movement in a ventilated case is controlled either by a mechanical ventilation system or through natural convection. Mechanical systems that use a low-air flow (cubic feet per minute - CFM) fan to force air through the case are usually unnecessary except in very large cases (more than 10 cubic yards in volume) or cases designed to maintain positive pressure. Carefully select and engineer the use of fans mounted in cases because they can cause excessive airflow, unacceptable vibration, and distracting noise. Air currents produced by a ventilation fan should not blow air onto objects.
A passive system using convection (air circulation due to a heat differences within the case) to move air through filtered vents is sufficient for most applications. The effectiveness of the passive ventilation system depends on the number, size, and location of vents in the case wall.
- Two vents are usually sufficient for small and medium-sized cases; large cases often require a series of vents.
- The vents need not be large; 4 cubic feet of air per minute can flow through a vent 1/4 inch in diameter.
The location and design of vents can promote the mixing of air throughout the case interior or can reduce air ventilation to a gradual circulation. Pockets of stagnant air in cases can allow volatile offgassing to concentrate and can create localized zones of different temperature and humidity inside the case. Therefore, locate vents to ensure sufficient circulation and avoid placing objects immediately next to a vent.
Vents installed laterally along intervals at the same height will restrict the "chimney effect", which is the rapid exhausting of air from a hole in the upper region of a space. Employing vents in a vertical series will encourage warmer air in the case to exit out the higher vents and cooler air to be drawn in at the lower ones. Whether using a series of vents or only two, they should be placed on opposing case walls or at large intervals on the same wall to minimize their height difference.
Ambient air must be filtered as it passes through an exhibit case. Dust filtration at the vents is particularly important when the overall room HVAC filtration is ineffective or nonexistent. Common case filtration methods include:
- fastening a prefabricated air duct filter to the inside of the vent, such as residential fiberglass filters, high efficiency particulate air (HEPA) filters, or filters containing pollution-absorbent products such as activated charcoal or a potassium permanganate-based product;
- fitting a vent with a commercial respirator filter, which eliminates particulate and gases; and
- covering the vent with a custom fabricated filter, such as a breathable fabric pouch that contains a commercially available pollution absorber.
Positive-Pressure Case Designs
Maintaining a positive pressure within an exhibit case is another option for controlling the entry of dust, insects, and buildup of exhibit case-generated chemical pollutants. When constructing a positive-pressure exhibit case system, no attempt is made to seal joints tightly.
One or more muffin fans are fitted into the case's exterior shell. Air from the overall exhibit space is circulated through a high-efficiency dust filter (a HEPA filter can be used) and into the case. Baffles inside the case can be fabricated so display objects are not affected by the air movement.
Positive-pressure cases are less expensive to construct than sealed or ventilated cases. They are practical for protecting objects that are susceptible to insect infestation and dust accumulation, such as dioramas or specimens and objects made of feathers, fur, or hair. Any changes in temperature and humidity levels, however, immediately affect the object contents of positive-pressure cases.
The objects must either be insensitive to the ambient changes, or the environmental control in the ambient exhibit space must be sufficient to maintain the conservation criteria established for the objects.
Lighting Design within Cases
- Develop an appropriate case lighting plan. Choose a lighting system that allows sufficient distance between lamps and separation panel and objects. Control heat buildup in the lighting chamber by using efficient, low-voltage systems, reducing lamp wattage, and, when necessary, using fans. Prototype and test all new or experimental lighting designs.
- Reduce heat gain and temperature cycling. Ventilate the lighting chamber to dissipate heat from fixtures and lamps. In larger cases or cases located in enclosed spaces, electric fans may be required. Heat gain inside the display chamber should be no more than 2°F when lights are turned on.
- Isolate lights from the display chamber. Place all lighting fixtures outside the display area of a case. Contain any lights that are integral to the case in a separate compartment. Seal off the lighting chamber to prevent the entry of insects, heat, and dust into the display chamber.
- Incorporate heat-reflecting and insulating materials when necessary. Consider heat-reflecting glass or double-glazed construction for panels that separate the lighting chamber. To help prevent heat buildup, use metal products to fabricate the lighting attic and insulate lighting compartments located below the display area.
The conservation concerns of exhibit case lighting must be addressed early in the design phase, not during installation.The design and exhibit lighting plan must address the following conservation issues:
- Selection of appropriate lighting equipment
- Sufficiently distancing the lighting sources from the objects to obtain lower illumination levels
- Preventing temperature and humidity destabilization within the display chamber
- Removal of damaging ultraviolet and infrared radiation
Exhibit case lighting is a preservation concern whether the lights are installed in a lighting chamber next to the displayed objects or are fixed to a track or ceiling system outside the case. The principal concern is the control of visible light, ultraviolet radiation, and case heating.
When integral lighting (case-mounted lighting, conventionally above the display ceiling) is called for, it is particularly challenging to obtain even lighting throughout a display chamber; the upper areas are usually overlit. This situation can result in uneven object lighting and localized damage to objects, especially when unfiltered fluorescent lamps are used.
A case designed to meet lighting conservation criteria will provide considerable flexibility for onsite adjustment and the ability to light at low levels. The greatest flexibility exists when the lighting is designed to provide a range of 5 to 40 footcandles (54 to 430 lux) in both the upper and lower regions of the case.
The location of lights and their heating potential affect the environmental conditions to which objects are exposed. The confined area of a display chamber is vulnerable to overheating if inappropriate fixtures and lamps are installed, particularly in well-sealed or hermetically sealed cases. Heat is generated by incandescent lamps, transformers used in low-voltage systems, and the ballasts of fluorescent lighting systems. Transparent glazing on a case can create a greenhouse effect, amplifying the heat passing into the chamber.
Improperly designed lighting raises the overall temperature in the case, which in turn destabilizes the interior relative humidity. The cycling of temperatures within a case creates a pressure differential that can draw air into the case as it cools when the lights are turned off. As a result, unconditioned air and dust will flow into the case. It is not uncommon for infrared radiation in exhibit lighting to actually heat the surfaces of display objects. In general, these problems are avoided if the heat gain within a case while lights are on is no more than 2°F.
Case-mounted lighting fixtures and lamps must be isolated in well-designed, separate lighting chambers, or attics. It is critical that lighting chambers be vented and serviced by a separate entry panel or door. Incorporate the following features when designing a lighting chamber:
- Well-sealed transparent separation panel between the lighting and display chambers that prevents air, insects, and dust from entering the display area
- Sealant applied around holes introduced into the display chamber, as with the use of fiber-optic lenses- also called luminaires
- Ability to add lighting filters and light-directing devices
- Chamber height that allows placement of lights at a sufficient distance from the separation panel
- Well-designed ventilation system that uses natural air convection and, when necessary, appropriately sized electric fans for exhausting heat from the lighting chamber
- Independent entry system with its own locking device
The lighting chamber must be tall enough to accommodate the lighting fixtures and filters and to allow adjustments in aim. Light-diffusing panels and light-directing equipment are critical for controlling the amount and type of light falling on light-sensitive objects. These light-modifying materials also serve aesthetic criteria by hiding insects and dust in the lighting chamber from view, preventing glare, and obscuring lighting hot spots. They require space and therefore have an impact on the design of the lighting chamber.
Both the design and the construction materials of the lighting chamber dramatically affect the heat gain in the display chamber below. The transparent panel between the two chambers can be made with heat-reflecting filters or glass, or it can be of double-glazed construction.
For the lighting chamber itself, metal is recommended over wood because it will dissipate heat. Well-designed vents of adequate size will also allow heat to dissipate rapidly. Exhaust fans may sometimes be required. Anticipate mechanical failure by providing alternative ventilation to avoid exposing objects to an increase in air temperature during any system malfunction.
Fixtures, Lamps and Light Modifying Materials
Choose lighting equipment for exhibit cases based on visible, ultraviolet, and infrared radiation characteristics; fire safety; and flexibility in focusing and illumination control during installation.
- Incandescent fixtures must have adjustable aim and accommodate lamps offering different beam widths and wattages. They should also accept a variety of light- controlling lenses, louvers, and baffles. Because dimmers and occupancy sensors are often required to fulfill conservation criteria, select only lighting systems that are compatible with this technology.
- Lighting manufacturers offer many types of compact fluorescent lamps that are very energy efficient, provide good diffuse light, and can be used successfully within cases. Pay particular attention to the lamp's UV output and filter it when necessary. Heat is generated by these lamps ballasts, therefore, each product and application must be considered independently.
- Fiber-optic lighting systems are also commonly used. They have the advantage of allowing the light source (or illuminator) to be located remotely, eliminating the problem of case heating. Through the selection of fiber type and their length both ultraviolet or infrared radiation can be controlled.
As a general guide, consider using a fiber-optic system in any of these situations:
- Ambient lighting is low and dramatic accent lighting is desired.
- Objects in close proximity have different lighting restrictions.
- Exposure to ultraviolet or infrared radiation can not be permitted.
- Localized, focused light beams are required.
- Concealed lighting sources and fixtures are desired.
- Energy efficiency is a primary concern.
- Optical fiber runs from the light source are less than 30 feet.
- Design a well-sealed case that will support humidity control. To achieve an effective microenvironment, minimize the air exchange between the case and the room. No more than 1 air exchange per 72 hours is recommended.
- Provide adequate air circulation within the case. If the environmental maintenance chamber is located beneath the objects use a perforated deck or a floating deck with a perimeter gap to allow air to circulate throughout both chambers.
- Provide separate access for maintenance. Climate-control equipment and materials in both active and passive systems will need to be maintained and adjusted.
- Test the case before enclosing objects. Ensure that the humidity inside the case meets the conservation criteria even when exterior conditions are at projected extremes.
Microenvironments within Exhibit Cases
A major benefit of a well-sealed case or frame is its ability to maintain a constant humidity level. Over the years, conservators and designers have developed simple, reliable, and cost-effective methods to protect humidity-sensitive collections on display in cases. Microclimates with specific environmental requirements can be provided on a case-by-case basis as a design response to stringent conservation criteria. Protection from airborne particulate and museum pests is also possible.
Stabilization vs. Control
Depending on the conservation criteria, a well-sealed enclosure can either stabilize or control its interior relative humidity. Stabilization evens out fluctuations in the relative humidity, reducing the rate and degree of change that may occur in an uncontrolled exhibit space. Alternatively, the RH control approach maintains a specific constant level of relative humidity.
The ambient moisture level of the room affects the relative humidity within an enclosure because of air diffusion, primarily through gaps in the case and secondarily through permeable exhibit case materials. These factors, discussed in a prior section Sealed Exhibit Cases, must be understood before designing an exhibit case to stabilize or control relative humidity. A well-sealed case will act to stabilize the interior relative humidity by slowing air-exchange rates.
Moisture exchange between hygroscopic components inside a case is an additional factor affecting internal humidity levels. Organic material, including display objects, wooden case elements, and paper or fabric liners, will release or absorb moisture in response to changes in the relative humidity or temperature within the case. This interplay can have a stabilizing effect on the relative humidity inside a case made of a significant quantity of hygroscopic materials. For this reason, cases are often loaded with cellulosic products and paper-based products which can be used for making mounts and interior case liners. Moisture absorbers such as silica gel are also commonly used.
Designing a Case for Humidity Control
A small volume of air is easier to control environmentally than a larger one. Therefore, a climate-controlled case should have the smallest possible ratio of air to objects. Microclimates in large walk-in cases are much more difficult to control than in small wall vitrines. The following features are generally required in a humidity-controlled case:
- a minimum air exchange between the case display chamber and the ambient environment
- a means of stabilizing or controlling humidity inside the display chamber-either an active (mechanical) system using humidifying/dehumidifying equipment or a passive system using silica gel, hygroscopic materials, or saturated salts
- adequate air mixing both within the display chamber and between it and the environmental maintenance chamber or climate-control equipment
- equipment to monitor internal relative humidity
- a means of reaching the mechanical or passive system for routine maintenance.
If the airflow within the case is restricted or pockets of still air exist, localized differences in relative humidity can develop. Air must circulate adequately between the display and maintenance chambers (or the environmental control equipment). Adequate air mixture can be accomplished by:
- using a perforated material (at least 40% open) to construct the display deck and covering it with fabric to form a platform on which objects can be arranged or mounted; or
- using a floating deck design that has a perimeter gap (generally 5/8 to 1 inch wide) to allow air circulation along all four sides.
The case design must provide for access to the maintenance chamber but limit the escape of conditioned air when the access panel is opened.
Testing and Monitoring
A humidity- or climate-controlled case should be tested before objects are enclosed in it. After testing, the relative humidity must be monitored continuously to alert museum staff to equipment malfunctions or the need for maintenance. A reliable, inexpensive monitoring device is a small thermo-hygrometer; hardwired monitors or data loggers offer the possibility of remote sensing.
Active and Passive Humidity-Control Systems
- Establish whether the goal is stabilization or control RH. Stabilizing the humidity inside a case is usually sufficient unless objects require a highly restrictive or specific RH range.
- Select an appropriate control method. Use mechanical systems cautiously, and choose specific equipment carefully. When using a passive system, design the case to include a holding area for the moisture-absorbent medium with easy access for installation and maintenance.
- Provide safeguards for mechanical systems. Locate equipment in a maintenance area that does not transfer heat or vibration to the objects. Provide a constant power supply (including emergency generators), a monitoring alarm to alert staff to equipment malfunction, and adequate water supply and drain lines.
- Include appropriate and sufficient moisture-absorbent materials for passive control. Systematically calculate the quantity and type of silica gel or cellulosic materials to be used.
- Test and monitor the case performance. Evaluate the initial performance of active or passive systems before enclosing objects. Monitor the relative humidity for the duration of the exhibit to alert staff when maintenance is required.
Active (Mechanical) Systems
Several commercial companies sell equipment to create and maintain a specific environment within exhibit cases. These machines, in effect, miniature HVAC systems, slowly feed conditioned air into the exhibit case and then either force the case air into the room or recirculate it. Commonly referred to as microclimate generators, such machines can provide a wide range of humidity levels more effectively than a building HVAC system and offer the possibility of refined relative humidity adjustment beyond that of a passive system.
Mechanical systems have been used with varying degrees of success. Most are expensive and not capable of compensating for the temperature swings that occur in an exhibit space. A water source and drain are required for humidification and dehumidification (to avoid requiring considerable staff maintenance). Water purity is an important issue as unfiltered water supplied to humidifiers can cause mechanical failure. The systems require considerable maintenance and introduce the risk of an electrical failure or equipment malfunction. Standby emergency generators are recommended to prevent damage from severe environmental swings during a power failure or mechanical downtime. The generators, in turn, create budget and storage issues. In addition, the relative humidity and temperature inside the case needs to be monitored continuously, and the monitoring system should be connected to an alarm that alerts staff of an equipment malfunction. These systems require considerable staff time.
Despite their drawbacks, mechanical systems are very useful in certain situations-for example, when the passive system will not meet the conservation criteria. A mechanical system can also be installed as a backup emergency system for a passive system. It would function when the ambient environment is too extreme for the passive system to accommodate or when the passive system fails.
If a mechanical system is desired, the conservator, designer, and facilities manager for the museum should be involved in researching and choosing a unit. There are also methods of controlling RH levels through the manipulation of room temperature.
In a passive environmental control system, a sealed case is designed to incorporate reactive materials that absorb and release moisture. The objective is either to stabilize (buffer) the enclosure against RH change or control (adjust) the interior RH to a specific level. Although these materials can both absorb and release moisture, they are known as moisture absorbers, and may be natural products (such as cotton or cellulose fiber) or man-made (such as silica gel).
Experience has shown that passive humidity control is very effective and can be incorporated into museum cases without major expense. Low maintenance and low initial expense makes passive designs especially attractive for museums with restricted exhibit budgets or for exhibits with many cases or cases in diverse locations. A passive system to stabilize or control the relative humidity inside a case requires:
- a well-sealed exhibit case-one with an air exchange rate of no more than 1 per 72 hours is recommended; a moderate-seal of 1 per 36 hours is less effective and requires much more absorber;
- a sufficient quantity of a moisture absorber;
- adequate air circulation over the absorber and between the display and maintenance chambers;
- a practical method of installing the absorber and periodically adjusting or replacing it; and
- a system to monitor the relative humidity inside the display chamber.
Three types of reactive materials are used in humidity- stabilized or humidity-controlled cases: silica gel, cellulosic materials and saturated salts. Use of these materials is discussed in more detail later in this section.
Stabilizing (Buffering) Humidity
Cases that buffer humidity are not engineered to provide a fixed or controlled relative humidity but to stabilize the interior environment from the more rapid changes occurring outside of the enclosure. In particular, a buffered case will even out daily cycles in interior relative humidity. This approach is practical for achieving a moderate relative humidity when humidity-sensitive objects can tolerate some degree of relative humidity fluctuation.
Reducing the effect of daily changes in relative humidity is an important conservation premise. Annual recording at a variety of museum sites has shown that in most exhibit spaces the daily fluctuations in relative humidity are too great (and too rapid) for the safe display of humidity-sensitive materials. However, even in museums with no humidity control the average relative humidity in any given year is usually within the acceptable range of 40 to 60% RH (outside of extremely arid or humid regions). The humidity levels in a buffered case are, by design, self-regulating between seasons. They will tend to reflect the yearly average of the whole exhibit space, avoiding exposure of the case contents to extreme conditions. (See Chart x)
Humidity-stabilized cases, if designed correctly, require little maintenance. These cases are practical in most situations. They require less staff maintenance than controlled cases, and they reduce the need for frequent entry. Enough of the moisture-reactive substance can be included in a case to buffer the internal environment throughout a three- to four-month season.
Even in exhibits with an RH differential of 30%, (where there is as much as 30% RH difference between the case and room) humidity-stabilized cases can be maintained for up to four months with only a 5 to 10% RH drift. Under the most extreme conditions, such as prolonged periods in which the room and case environments differ by more than 30% RH, it may be necessary to add a small amount of moist or dry absorber to boost the stabilizing absorber back to the acceptable range.
Controlling (Adjusting) Humidity
In contrast to humidity-stabilized cases, humidity-adjusted cases aim to maintain an exact relative humidity for the duration of an exhibit. This type of case has been the subject of considerable experimentation by museums.
Traditionally, a moisture absorber such as silica gel is conditioned to a specific RH and enclosed in a tightly sealed case. The gel requires reconditioning when it can no longer maintain the case atmosphere at the intended relative humidity level. Reconditioning and handling of the silica gel requires considerable staff time and training (few automated systems are commercially available for conditioning the gel in place). A particular challenge of these case designs is the environmental fluctuation to which the objects are exposed when a case is opened to recondition the gel. When possible, the case design should be elaborate enough to allow the gel to be reconditioned without opening the main case door and producing a total air exchange.
Moisture Absorbers Used in Cases
Silica gel is very effective at regulating relative humidity because of its large surface area, which allows large quantities of moisture to be absorbed or adsorbed. Silica gel beads or pellets can be placed in containment trays or held in bags made of synthetic fiber fabrics (such as quilted nylon mesh bags that weigh one pound or more).
Cassettes, panels, and tiles of silica gel are also available (or can be fabricated by museum staff), as are papers and foams impregnated with hybrid gels. Hybrid gels developed during the last decade, however, are better at maintaining an RH of between 40 and 60%. Traditional silica gel works well as a desiccant and can efficiently maintain a relative humidity of between 30 and 40%.
Silica gel function is improved by spreading the gel out over a large area which serves to increase the exposed surface area of the gel. Loose silica gel beads should be held in shallow beds, 1 to 2 inches deep so that air can easily penetrate the gel. Tiles filled with gel should stand on end when possible to increase air-to-gel contact. The quantity of moisture absorber required in a specific case depends on these factors:
- Type of absorber (e.g. silica gel)
- Volume of the display chamber and the permeability of its materials
- Degree of seal in the display and maintenance chambers
- Moisture-reactive qualities of the case and its contents
- Projected differential between the desired interior humidity and exterior environment
- Temperature fluctuation within the case
- Length of time the case is to perform without maintenance
- Degree of air circulation between the display and maintenance chambers
Some degree of experimentation will be necessary to determine the quantity of silica gel. The quantities of absorber required must be related to the type of cases designed and the degree of seal achieved. A rule of thumb for a well-sealed case is 1/4 pound of hybrid silica gel to buffer 1 cubic foot of air- an moderately-sealed case requires at least twice the quantity of gel. An unsealed case cannot provide any climate control even when a tremendous amount of gel is included.
Using Cellulosic Materials and Saturated Salts
Cellulosic materials placed inside a case can provide a practical, low-cost means of stabilizing relative humidity. Any hygroscopic cellulosic material-including cotton, wood, or paper (sheet, paperboard, or honeycomb panels)-will absorb or release moisture in response to changes in relative humidity. A case loaded with such materials will experience a more stable relative humidity than that of the ambient exhibit environment. However, these materials are often not as efficient at controlling atmospheric moisture in the case and may introduce subsequent problems and contaminants such as acidity and mold infestation. Large quantities of these natural buffers are required, making them less efficient than silica gel for use in cases having large air volumes.
Saturated salts, although they have been used effectively in the past, have been largely superseded by easier-to-use silica gel. There are logistical complications in case design and maintenance when using saturated salts. In addition, salts require special handling, and they tend to seep over the edges of conventional containers. This can result in contamination of display objects. Salts can be useful, however, in conditioning silica gel outside cases.
- Incorporate enough absorber to remove pollutants for six months to one year. Objects must never touch a chemical absorber.
- Ensure unrestricted airflow. Case design should encourage air movement across the surface of the pollutant absorber. Ensure that the case is well-sealed.
- Provide access to change the absorber. A small access port can serve both moisture and pollutant absorbers.
- Maintain the absorber. Renewal of activated charcoal is critical to prevent secondary off-gassing. To ensure continual filtration, both activated charcoal and potassium permanganate must be replaced when exhausted.
The two most common pollution absorbers used in exhibit cases are activated charcoal and potassium permanganate. Acid-free, alkaline reserve papers or boards included in a frame can provide similar filtering benefits because the paper will absorb pollutants as the air passes through the paper fibers. The effectiveness and longevity of this technique are, however, difficult to quantify. (See TechNotes)
Charcoal-based products are sold in two formats: loose pellets that can be contained in perforated panels or fabric mesh bags, and paper- or fabric-based sheets embedded with activated charcoal. Activated charcoal does not react with contaminants but simply absorbs a range of them. The charcoal becomes ineffective when it has taken up its maximum amount of pollutant, and the pollutant-loaded charcoal can become a secondary source of contamination as pollutants dissipate again into the case atmosphere.
Therefore, it is critical to replace charcoal-based absorbers before they become exhausted: a point that is difficult to determine, since there is no visual change in the material. An appropriate amount of absorber, based on the case size, rate of air leakage, and concentration of pollutants, must be changed at predetermined intervals of usually no more than 12 months.
Despite the maintenance issues involved in using activated charcoal products, they remain an excellent choice for exhibit cases due to their low cost and superior performance. In several studies, activated charcoal proved most effective among a range of absorbers for removing ozone, sulfur dioxide, nitrogen dioxide, hydrogen sulfide, and formaldehyde.
Potassium permanganate acts as a pollutant scavenger and has been successfully used in exhibit cases. It is commercially available in HVAC filters or as loose pellets. Benefits of potassium permanganate over the less expensive activated charcoal include:
- reacting with pollutants, thus preventing any seconday offgassing of pollutants from the absorber; and
- changing color from pink to brown as it becomes exhausted, providing an easy way of determining when it must be replaced.
Use in Exhibit Cases
Whether using a charcoal or potassium permanganate product, a case built for pollution control is similar to one constructed for relative humidity control. It must be as airtight as possible to minimize the amount of absorber needed and improve the efficiency of pollutant removal. Place the pollutant absorber in the case in one of several ways:
- held contained in a contaminant tray in the maintenance chamber beneath the display deck
- hidden behind large objects
- used as an impregnated fabric covering the case floor, walls, or shelves
- enclosed in a frame package
Use a sufficient quantity of the pollution-control substance. Cloth and paper liners placed in the display chamber may be sufficient for short exhibits of three to six months, but this method is less appropriate for extended exhibits because replacing the cloth or paper is disruptive. For longer exhibits, the activated charcoal or potassium permanganate should be contained in the maintenance chamber or under the deck; access must be provided to replenish the absorber.
When a layer of cloth or paper is included in the display chamber, an isolating layer or an object mount is required to prevent direct contact between the objects and the absorber. In all applications, greater surface area of the pollutant absorber increases the effectiveness of absorption.
Unrestricted airflow inside the case ensures that air in the display chamber passes over the absorber (See Technical Illustrations). Although a fan or other mechanical air circulation system achieves the most effective circulation, experience has proven that convection is usually sufficient. Mechanical systems are sometimes necessary in oversized cases and in instances where considerable air pollution is anticipated.
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Pages in category "Exhibit Case Design"
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